Power amplifiers for wireless communication applications are often subjected to elevated voltages and extreme voltage standing wave ratio (VSWR) output loads. These power amplifiers are preferably constructed using an array of Gallium Arsenide or like heterojunction bipolar transistors (HBTs), which are known to break down under such elevated voltages and extreme VSWR loads. For example, the base collector breakdown voltage for a two micron HBT process is approximately 24 volts, wherein the collector emitter breakdown voltage is significantly less, at approximately 14 volts. Even when power supply voltages are significantly less than either of these breakdown voltages, leakage currents entering the base regions of the transistors experience beta multiplication if they are not provided with a good path to ground through the base contact.
Breakdown problems typically occur during portions of the radio frequency cycle just before and after transistor conduction. When the output device is not conducting and the collector current is close to zero, the collector voltage exhibits a “ringing” phenomenon similar to an under damped step response. The waveform for the collector voltage is influenced by a number of factors, such as: the matching network, the compression state of the amplifier, the amplifier bias, the low output impedance at the output of the amplifier, and the termination of any harmonics. The peak voltages at the collector will often exceed twice the power supply voltage under normal conditions even when the collector current is close to zero. Under extreme VSWR load conditions and at certain load reflection coefficient phase angles, the voltages can exceed three times the supply voltage, which may result in device failure.
Certain extreme VSWR load reflection coefficient phase angles produce very low impedances at the amplifier output. These low output load impedances produce large amplifier currents, which may lead to failure or output waveform distortion by spectral re-growth when operated in a pulsed mode. Amplifier failure occurs when an extreme VSWR load induces large amplifier current, which exceeds the safe-operating-region of the HBT devices. Amplifier distortion due to spectral re-growth in a pulsed application is caused by the inherent self-heating characteristic of HBT devices. The self-heating nature of the HBT devices creates a current gain reduction with increased DC power dissipation or device temperature. This self-heating induced current gain reduction creates a time variant HBT current gain function during pulsed operation. A time variant transfer function of the HBT devices modulates the input signal causing spectral spreading or re-growth centered on the amplified output RF signal.
A user simply touching the antenna of a mobile telephone may induce the output load mismatches that can lead to such conditions. Essentially, the act of touching the antenna changes the output load impedance for the device, which may lead to extreme VSWR conditions and excessive voltage peaks on the collectors of the output transistors. Given the relative ease in which such conditions are induced, there is a need for a way to protect the power amplifier circuitry from these conditions in an efficient and cost-effective manner.